Method for producing (meth)acrylic acid esters

ABSTRACT

Higher (meth)acrylates IV are prepared by transesterification of a lower (meth)acrylate I with a higher alcohol R 2 OH in the presence of a stabilizer or stabilizer mixture and of a catalyst or catalyst mixture, by a process in which the liberated lower alkanol R 1 OH is separated off and is fed at least partly to the preparation of the lower (meth)acrylate I.

The present invention relates to a process for the preparation of higher(meth)acrylates by esterification of a lower (meth)acrylate with ahigher alkanol and recycling of the liberated lower alcohol to theprocess for the preparation of the lower (meth)acrylate.

(Meth)acrylates are useful starting compounds for the preparation ofpolymers and copolymers which are used, for example, as finishes,dispersions or adhesives.

In this application, the terms (meth)acrylic acid and (meth)acrylaterefer to methacrylic acid and acrylic acid, and methacrylate andacrylate, respectively.

Kirk Othmer, Encyclopedia of Chemical Technology, 4th Ed., 1994, pages301–302, describes a preparation of lower acrylates by esterification,in which acrylic acid, alkanol and catalyst, e.g. sulfuric acid, arereacted using recycled streams in a reactor with connected distillationcolumn, in which the desired ester, excess alkanol and the water formedduring the reaction are separated off via the top. The phases in thedistillate are separated and a part of the organic phase is added asreflux to the distillation column, but the major part is fed to a washtower in which alkanol and acrylic acid are removed by washing. Ifrequired, base may be added to this wash process to remove traces ofacrylic acid. The water-containing ester is then freed from water bydistillation and purified by distillation in a further distillationcolumn, and once again the phases are separated in the distillate. Thehigh boilers remaining behind are passed into a stripping apparatus, anduseful products are separated off and recycled.

The aqueous phases obtained in the process and originating from thedistillate in the elimination of water, the refined fraction in thewashing and the aqueous phase of the distillate of the distillationcolumn connected to the reactor are combined and the alkanol andacrylate contained therein are separated off in an alcohol stripper andare recycled to the reactor.

Furthermore, high boilers from the esterification reactor are removed,and useful products are distilled off in a stripping apparatus and arerecycled to the esterification reactor.

In the case of the esterification with butanol, the alkanol is removedas an azeotropic butanol/butyl acrylate mixture using a furtherazeotrope column and is recycled to the reactor.

In Ullmann's Encyclopedia of Industrial Chemistry, 6th ed, 1999Electronic Release, Chapter: Acrylic Acid andDerivatives—Esterification, a process for the preparation of higheralkyl acrylates is described which is carried out in the presence of anorganic solvent as an entraining agent and sulfuric acid as a catalyst.The water formed in the reaction is removed by means of azeotropicdistillation.

The reaction mixture removed from the reactor is neutralized with alkaliand separated into an aqueous phase and an organic phase. The organicphase is freed from the organic solvent used and from the alkanol in twodistillation columns, and the crude ester thus obtained is then purifiedby distillation.

The aqueous phases obtained in the process are combined, and organicmaterial contained therein is separated off and is recycled to thereaction.

From these two processes, it is evident that (meth)acrylic acid can beseparated from the corresponding ester by treatment with a base.

DE-C2 232 33 28 describes a process for the extractive separation of theunconverted acrylic acid from the esterification waste liquors which areobtained in the esterification of acrylic acid with alkanols, such asn-butanol, isobutanol and 2-ethylhexanol, by extraction with analkanol/alkyl acrylate mixture.

The aqueous solutions which are fed to the extraction are said tocontain the acrylic acid in free form, i.e. alkaline or neutral wasteliquors are expediently acidified, for example with hydrochloric orsulfuric acid, before the extraction, so that all of the acrylic acid isliberated.

When the process is carried out in practice, the amount of butanol/butylacrylate mixture relative to aqueous acrylic acid solution can be variedwithin wide ranges. Butanol/butyl acrylate mixtures of said type extractacrylic acid far more effectively than do butyl acrylate and butanol bythemselves.

In summary, the esterification of (meth)acrylic acid with an alkanol of1 to 8 carbon atoms is generally acid-catalyzed, and the catalysts usedare, for example, acidic or strongly acidic ion exchangers, sulfuricacid or sulfonic acids, e.g. para-toluenesulfonic acid, methanesulfonicacid, trifluoromethanesulfonic acid, benzenesulfonic acid,xylenesulfonic acid, naphthalenesulfonic acid or dodecylbenzenesulfonicacid. Up to 20% by weight of catalyst may be used, cf. for example DE-A195 10 891.

The (meth)acrylic acid used can, for example, be purified beforehand,but it may also be a crude (meth)acrylic acid, as described, forexample, in DE-A 198 51 983. A cascade may be an expedient reactor, cf.for example DE-A 195 36 178.

The water of reaction formed during the reaction is generally separatedoff by distillation, with or without the addition of an entrainingagent, e.g. benzene, toluene or cyclohexane, which forms an azeotropicmixture with water, if required as an azeotropic mixture with thealkanol used. Frequently, however, the desired ester is distilled offtogether with the water of reaction formed during the reaction and thealkanol, cf. for example WO 99/23060 or U.S. Pat. No. 4,280,010.

As described above, the distillate which frequently contains(meth)acrylic acid is generally separated into an organic phase and anaqueous phase.

The organic phase can be recycled partly or substantially completely asa reflux to the distillation column, but it is generally subjected atleast partly to a separation operation in which the starting alkanol isseparated from the desired ester, it being possible for the desiredester to be further purified if necessary and the alkanol to be recycledto the reaction.

The aqueous phase can be recycled at least partly as a reflux to thecolumn, but it may also be directly discharged or subjected to a furtherworking-up in which useful products present are separated off.

The reaction mixture removed from the reaction zone is generally fed toa separation zone which usually has at least one rectification unit. Ifrequired, extraction is effected beforehand with a suitable solvent,e.g. water, in order to separate off (meth)acrylic acid and/or catalyst.In the separation zone, further phase separations between an organicphase and an aqueous phase can also be carried out, as described, forexample, in DE-A 196 04 252 and DE-A 196 04 253.

Oligomers (oxyesters) obtained as high boilers in the course of thepreparation and/or working-up process can be cleaved, for examplethermally and in the presence of stabilizers, as described in U.S. Pat.No. 3,868,410, or in the presence of acid, e.g. dodecylbenzenesulfonicacid, cf. for example WO 00/27789, in the presence of (meth)acrylic acidor oligomeric (meth)acrylic acid, cf. for example DE-A 195 47 485 andDE-A 195 47 459, and, if required, additionally in the presence ofwater, cf. for example DE-A 197 01 737. This cleavage can be carriedout, for example, in a reaction cascade, cf. for example CN 1 058 390and CN 1 063 678.

The preparation of (meth)acrylates by transesterification in thepresence of acidic or basic catalysts is generally known. Catalystsproposed in particular are titanium alcoholates whose alkyl groups areC₁–C₄-alkyl radicals, e.g. tetramethyl, tetraethyl, tetraisopropyl,tetra-n-propyl, tetraisobutyl and tetra-n-butyl titanate (cf. forexample EP-B1 298 867, EP-A2 960 877). Other proposed catalysts includetitanium phenolates (German Laid-Open Application DOS 20 08 618), metalchelate compounds of, for example, hafnium, titanium, zirconium orcalcium, alkali metal and magnesium alcoholates, organic tin compoundsor calcium and lithium compounds, for example oxides, hydroxides,carbonates or halides.

Since the transesterification is known to be an equilibrium reaction,one of the starting materials must be used in a large excess and/or oneof the reaction products must be removed from the equilibrium in orderto obtain economical conversions. As a rule, the lower alkanol R¹OH (cf.equation 1) liberated during the transesterification, being the alcoholcomponent having the lowest boiling point, is therefore removed from theequilibrium by distillation. The disadvantage here is that the liberatedalkanols, usually methanol or ethanol, form an azeotropic mixture withthe corresponding (meth)acrylates (methyl or ethyl (meth)acrylate) andhence cannot be separated directly by distillation.

In addition, the distillate contains at least traces of the higheralcohol R²OH and consequently also cannot be recycled directly to theprocess for the preparation of the lower (meth)acrylate I of the loweralkanol R¹OH.

For ecological and economic reasons, however, the reuse of the mixtureor azeotropic mixture distilled off or of its individual components(alkanol and/or (meth)acrylate) is advantageous.

Owing to the position of the boiling points and/or the formation ofazeotropic mixtures, this distillate, as stated above, generally doesnot consist of the pure lower alkanol but is contaminated with the lower(meth)acrylate and possibly the higher alcohol.

In the case of the preparation of dimethylaminoethyl acrylate fromn-butyl acrylate and dimethylaminoethanol, for example, a distillatewhich predominantly comprises 5–15% by weight of n-butyl acrylate,85–95% by weight of n-butanol and 0.01–0.5% by weight ofdimethylaminoethanol is obtained.

The individual components have the following boiling points (bp.):

n-Butyl acrylate bp. 146.7° C. n-Butanol bp. 117.5° C. n-Butanol/n-butylAcrylate azeotrope bp. 117° C. Dimethylaminoethanol bp. 133.9° C.

Since, for the above reasons, it is desirable to utilize the distillate,contamination has an adverse effect, particularly if, as in this case,the higher alcohol is a basic compound, i.e. here an amino group. Thedirect recycling to the synthesis of the lower ester, which iseconomically particularly desirable, is thus adversely affected inparticular (EP-A 906 902, page 3, lines 4 to 16).

Owing to the small boiling point difference or azeotrope formation, someof the impurities are difficult to remove and can lead to the formationof further byproducts, for example by esterification reactions oraddition of the double bond of the esters.

EP-A 906 902 attempts to solve the problem caused by the basicimpurities by passing the alcohol-containing distillate, either directlyor after an additional distillation, over an acidic ion exchange resin.The basic, nitrogen-containing impurities are bound by the acidic groupsand thus separated from the alkanol/acrylate mixture.

The process described in EP-A2 906 902 for the preparation and isolationof alkylaminoalkyl (meth)acrylates substantially comprises the followingstages:

-   1. Batchwise transesterification in the presence of the catalyst    dibutyltin oxide and of the stabilizer phenothiazine, the main    amount of the alkylaminoalkanol being added after the start of the    reaction in a manner such that its concentration in the reaction    mixture does not exceed 25 mol %.-   2. Distillative removal of the lower alkanol formed in the    transesterification, as an azeotropic mixture with the lower    (meth)acrylate via a column, it being possible, if necessary, for    the distillate to be subjected to a further distillation.-   3. Treatment of the distillate, which mainly comprises lower alkanol    and lower (meth)acrylate, with an acidic cation exchanger. The basic    impurities (amines) which prevent the use of the distillate in the    preparation of the lower ester due to deactivation of the catalyst    used, are separated off thereby.-   4. Distillative separation of the reaction mixture in the    transesterification into a top product, mainly comprising desired    ester, lower alkanol and starting materials, and a bottom product,    which substantially contains catalyst, stabilizer, Michael adducts    and polymers and which may be reusable in the transesterification.    If the catalyst loses its activity, it is disposed of.

Alternatively, the catalyst can be separated off in two stages, firstthe lower (meth)acrylate being separated off via the top of a column andrecycled to the transesterification. In a second distillation column,the desired ester and remaining low boilers are separated off as topproduct, and the catalyst-containing bottom product is, if required,reused in a transesterification.

-   5. The distillate containing the desired ester is separated, in a    further distillation stage, into a top product containing    aminoalkanol and lower ester, which can be reused in the    transesterification, and a bottom product which contains the desired    ester.-   6. Finally, in a further distillation step (purification by    distillation), the desired ester is isolated in a purity of 99.8%    from the bottom product containing the desired ester.-   7. A part of the desired ester is obtained from the bottom product    of the purification by distillation, which still contains the    desired ester, in a distillation, preferably a thin-film    distillation, and is fed to the low boiler distillation.

Among the disadvantages of the process are

-   -   that the transesterification is carried out batchwise,    -   that the dialkylaminoalkanol has to be added to the reactor over        a long period (4 hours),    -   that long reaction times (7–8 hours) are required, favoring the        formation of byproducts and of polymer,    -   that the azeotropic mixture has to be purified by a technically        complicated procedure over an ion exchanger bed, which is        environmentally polluting owing to the necessity of rinsing,    -   that the yield is low (about 33%, based on dimethylaminoethanol        used, cf. example III-1 in EP-A2 906 902) and    -   that the residues are not worked up to recover useful products.

EP-A2 960 877 describes a continuous process for the preparation ofdialkylaminoalkyl (meth)acrylates by transesterification of methyl orethyl (meth)acrylate with dialkylaminoalkanols in the presence oftetraethyl, tetrabutyl or tetra(2-ethylhexyl) titanate. Thetransesterification there is carried out in a stirred reactor and theworking-up of the reaction mixture is effected in the following steps:

-   1. The reaction mixture is separated in a distillation unit into a    top product, which substantially contains the desired ester and the    low boilers, and a bottom product, which mainly comprises high    boilers, catalyst and a little desired ester.-   2. The bottom product can, if required, be purified in a thin-film    evaporator, the distillate being recycled to the    transesterification. The catalyst-containing bottom product is    discharged.-   3. The top product containing the desired ester is separated in a    further distillation unit into a low boiler fraction, which is    recycled to the reactor, and a bottom product, predominantly desired    ester.-   4. In a further distillative purification step, the desired ester is    isolated (purity 99.8%) as top product from the bottom product. The    resulting residue is recycled to the low boiler removal.

This process has, inter alia, the following disadvantages:

-   -   The transesterification is carried out in a stirred reactor        which is likely to need repairs owing to its moving parts    -   The alkanol component of the catalyst leads to impurities (cf.        EP-A2 960 877, page 2, lines 49 to 50)    -   No utilization of the distillate separated off in the reactor        and hence a loss of useful products    -   No utilization of the resulting high boilers (e.g. Michael        adducts)

In order to avoid in general the formation of a distillate or azeotropicmixture which consists of the lower alkanol and the corresponding(meth)acrylate in the transesterification, various patents (e.g. U.S.Pat. No. 2,406,561, German Laid-Open Application DOS 2 145 283, EP-B1210907) propose the use of assistants which form heteroazeotropes withthe lower alkanols liberated, e.g. hexane, cyclohexane or benzene.

The lower alkanol liberated during the transesterification is separatedoff by distillation as an azeotropic mixture with the assistant, thecondensate being separated into two phases. The phase which contains theassistant is recycled to the transesterification and the alkanol phasesaturated with assistant is discharged. However, this alkanol phase mustbe separated from the residual assistant before further utilization.German Laid-Open Application DOS 2 145 283 proposes, for example, theseparation of the benzene/alkanol azeotropic mixture by means ofmolecular sieves. Such a process is complicated and is not economical onan industrial scale.

DE-A 23 17 226 proposes separating the azeotropic mixture formed fromalkanol and the corresponding (meth)acrylate by treatment with water,the alkanol being washed out. The process is not economical since anaqueous alkanol solution forms and has to be disposed of or worked up,and the ester phase has to be dried before recycling to thetransesterification.

EP-A2 143 639 recommends the separation of these azeotropic mixturesusing complex-forming salts, e.g. LiCl, and an extracting agent. Theprocess is uneconomical since it produces wastewater and requires aplurality of distillation steps.

EP-A1 736 510 proposes carrying out the separation of the azeotropicmixture comprising methyl (meth)acrylate and methanol and possibly waterby distillation in the presence of a solvent which forms an azeotropicmixture with methanol, e.g. pentane, hexane, heptane or2,3-dimethylbutane.

However, the use of an additional assistant makes this process, too,uneconomical.

Transesterification processes in which no azeotropic mixture is formedhave also been proposed.

EP-A2 160 427 proposes, for example, a transesterification in theabsence of free higher alcohol. There, the lower (meth)acrylate isreacted with the titanium alcoholate of the higher alcohol, the titanateof the lower alkanol being formed in addition to the desired ester andbeing reacted in a separate reaction step with the higher alcohol togive the corresponding titanate again. Because of the large amounts oftitanate required, this process is of no commercial importance.

A further problem in the transesterification is the formation of Michaeladducts. Here, Michael adducts are understood as meaning the compoundsformed by addition of the alcohols at the double bond of the(meth)acrylates (EP-A2 906 902, pages 8 to 9).

It is generally known that this addition (cf. equation 2) takes place inparticular in the presence of alkaline catalysts (Organikum, 17thEdition, page 506, VEB Deutscher Verlag der Wissenschaften, Berlin1988).

In the transesterification according to equation 1, the adducts (II) and(III) play a substantial role

The results of this adduct formation are lower yields and a morecomplicated distillation for obtaining the desired ester in high purity.

The formation of the adducts according to the general equation 2 can, asis generally known, be reduced by minimizing the concentration of freealkanol. EP-A2 906 902 therefore proposes continuously adding the mainamount of the alkanol during the transesterification and not allowingthe concentration of free alkanol to increase above 25 mol %.

It is an object of the present invention to provide a process for thepreparation of higher (meth)acrylates by transesterification of lower(meth)acrylates, in which the lower alcohol liberated can be reused inthe preparation of the lower (meth)acrylate without additional processengineering steps.

We have found that this object is achieved and that higher(meth)acrylates IV can be prepared by transesterification of a lower(meth)acrylate I with a higher alcohol R²OH in the presence of astabilizer or stabilizer mixture and of a catalyst or catalyst mixture,if the lower alkanol R¹OH liberated is separated off and is at leastpartly fed to the preparation of the lower (meth)acrylate I.

The lower alkanol R¹OH liberated during the transesterification can befed to the preparation of the lower (meth)acrylate I without furtherpurification.

It is preferably used for reextraction of (meth)acrylic acid fromaqueous phases obtained in the working-up process, particularlypreferably in the aqueous phase which is obtained by separating off theexcess (meth)acrylic acid from the esterification mixture.

The lower alkanol R¹OH liberated during the transesterification ispreferably separated off by distillation.

The novel process is generally carried out as follows:

R¹, R² and R³ may each be aromatic, aliphatic or cycloaliphatic,straight-chain or branched, saturated or unsaturated and may containheteroatoms or aromatic substituents.

The radicals preferably comprise:

-   R¹=C₁–C₄-alkyl-   R=H or CH₃-   R²=C₂–C₁₂-alkyl, substituted by at least one NR³ ₂ group, where R³    may be identical or different.-   R³=C₁–C₆-alkyl, where N may also be a member of a five- to    seven-membered ring.

R¹ should contain at least one carbon atom less than R².

R¹ is, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, 2-butyl or tert-butyl, preferably n-butyl or isobutyl.

R² is, for example,

2-(dimethylamino)ethyl, 3-(dimethylamino)propyl, 4-(dimethylamino)butyl,5-(dimethylamino)pentyl, 6-(dimethylamino)hexyl, 8-(dimethylamino)octyl,10-(dimethylamino)decyl, 12-(dimethylamino)dodecyl,2-(diethylamino)ethyl, 3-(diethylamino)propyl, 4-(diethylamino)butyl,5-(diethylamino)pentyl, 6-(diethylamino)hexyl, 8-(diethylamino)octyl,10-(diethylamino)decyl, 12-(diethylamino)dodecyl,2-(di(isopropyl)amino)ethyl, 3-(di(isopropyl)amino)propyl,4-(di(isopropyl)amino)butyl, 5-(di(isopropyl)amino)pentyl,6-(di(isopropyl)amino)hexyl, 8-(di(isopropyl)amino)octyl,10-(di(isopropyl)amino)decyl, 12-(di(isopropyl)amino)dodecyl,2-(dibutylamino)ethyl, 3-(dibutylamino)propyl, 4-(dibutylamino)butyl,5-(dibutylamino)pentyl, 6-(dibutylamino)hexyl, 8-(dibutylamino)octyl,10-(dibutylamino)decyl, 12-(dibutylamino)dodecyl, 2-(dihexylamino)ethyl,3-(dihexylamino)propyl, 4-(dihexylamino)butyl, 5-(dihexylamino)pentyl,6-(dihexylamino)hexyl, 8-(dihexylamino)octyl, 10-(dihexylamino)decyl,12-(dihexylamino)dodecyl, 2-(methylethylamino)ethyl,2-(methylpropylamino)ethyl, 2-(methylisopropylamino)ethyl,2-(methylbutylamino)ethyl, 2-(methylhexylamino)ethyl,2-(methyloctylamino)ethyl, 2-(ethylpropylamino)ethyl,2-(ethylisopropylamino)ethyl, 2-(ethylbutylamino)ethyl,2-(ethylhexylamino)ethyl, 2-(ethyloctylamino)ethyl,3-(methylethylamino)propyl, 3-(methylpropylamino)propyl,3-(methylisopropylamino)propyl, 3-(methylbutylamino)propyl,3-(methylhexylamino)propyl, 3-(methyloctylamino)propyl,3-(ethylpropylamino)propyl, 3-(ethylisopropylamino)propyl,3-(ethylbutylamino)propyl, 3-(ethylhexylamino)propyl,3-(ethyloctylamino)propyl, 4-(methylethylamino)butyl,4-(methylpropylamino)butyl, 4-(methylisopropylamino)butyl,4-(methylbutylamino)butyl, 4-(methylhexylamino)butyl,4-(methyloctylamino)butyl, 4-(ethylpropylamino)butyl,4-(ethylisopropylamino)butyl, 4-(ethylbutylamino)butyl,4-(ethylhexylamino)butyl, 4-(ethyloctylamino)butyl,2-(N-piperidinyl)ethyl, 3-(N-piperidinyl)propyl, 4-(N-piperidinyl)butyl,5-(N-piperidinyl)pentyl, 6-(N-piperidinyl)hexyl, 8-(N-piperidinyl)octyl,10-(N-piperidinyl)decyl, 12-(N-piperidinyl)dodecyl,2-(N-pyrrolidinyl)ethyl, 3-(N-pyrrolidinyl)propyl,4-(N-pyrrolidinyl)butyl, 5-(N-pyrrolidinyl)pentyl,6-(N-pyrrolidinyl)hexyl, 8-(N-pyrrolidinyl)octyl,10-(N-pyrrolidinyl)decyl, 12-(N-pyrrolidinyl)dodecyl,2-(N-morpholino)ethyl, 3-(N-morpholino)propyl, 4-(N-morpholino)butyl,5-(N-morpholino)pentyl, 6-(N-morpholino)hexyl, 8-(N-morpholino)octyl,10-(N-morpholino)decyl, 12-(N-morpholino)dodecyl,2-(N′-methyl-N-piperazinyl)ethyl, 3-(N′-methyl-N-piperazinyl)propyl,4-(N′-methyl-N-piperazinyl)butyl, 5-(N′-methyl-N-piperazinyl)pentyl,6-(N′-methyl-N-piperazinyl)hexyl, 8-(N′-methyl-N-piperazinyl)octyl,10-(N′-methyl-N-piperazinyl)decyl, 12-(N′-methyl-N-piperazinyl)dodecyl,2-(N′-ethyl-N-piperazinyl)ethyl, 3-(N′-ethyl-N-piperazinyl)propyl,4-(N′-ethyl-N-piperazinyl)butyl, 5-(N′-ethyl-N-piperazinyl)pentyl,6-(N′-ethyl-N-piperazinyl)hexyl, 8-(N′-ethyl-N-piperazinyl)octyl,10-(N′-ethyl-N-piperazinyl)decyl, 12-(N′-ethyl-N-piperazinyl)dodecyl,2-(N′-isopropyl-N-piperazinyl)ethyl,3-(N′-isopropyl-N-piperazinyl)propyl,4-(N′-isopropyl-N-piperazinyl)butyl,5-(N′-isopropyl-N-piperazinyl)pentyl,6-(N′-isopropyl-N-piperazinyl)hexyl,8-(N′-isopropyl-N-piperazinyl)octyl,10-(N′-isopropyl-N-piperazinyl)decyl or12-(N′-isopropyl-N-piperazinyl)dodecyl.

Furthermore, R²OH may be ethoxylated and/or propoxylated alcohols ormixed ethoxylated/propoxylated aminoalcohols

-   R³ ₂N(—CH₂CH₂—O)_(y)—H or-   R³ ₂N(—CH(CH₃)—CH₂—O)_(y)—H or R³ ₂N(—CH₂—CH(CH₃)—O—)_(y)—H,-   where y is an integer from 1 to 4.

Dialkylaminoethanols are preferably used, dimethylaminoethanol,diethylaminoethanol and di-n-butylaminoethanol being particularlypreferred.

The transesterification of the lower (meth)acrylate (I) with the higheralkanol R²OH is carried out in the presence of a catalyst or a catalystmixture in a manner known per se, for example by one of the processesmentioned at the outset.

Typical conditions under which the transesterification can take placeare, for example:

lower (meth)acrylate:higher alcohol R²OH = 1–4:1 (molar) Amount ofcatalyst in the reaction mixture = 0.5–5% by weight Amount of stabilizerin the reaction mixture = 0.05–0.5% by weight Reaction temperature =60–160° C. Duration of reaction = 1–10 hours

The reaction can be carried out under atmospheric, superatmospheric orreduced pressure, preferably at atmospheric pressure or under slightlyreduced pressure (300–800 mbar absolute).

The transesterification can be carried out, for example, continuously,semicontinuously or batchwise, preferably continuously.

Examples of stabilizers used are N-oxyls, e.g.4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl or4-oxo-2,2,6,6-tetramethylpiperidin-N-oxyl, phenols and naphthols, e.g.p-aminophenol, p-nitrosophenol, 2-tert-butylphenol, 4-tert-butylphenol,2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol,4-methyl-2,6-di-tert-butylphenol (2,6-di-tert-butyl-p-cresol) or4-tert-butyl-2,6-dimethylphenol, quinones, e.g. hydroquinone orhydroquinone monomethyl ether, aromatic amines, e.g. N,N-diphenylamine,phenylenediamines, e.g. N,N′-dialkyl-para-phenylenediamine, it beingpossible for the alkyl radicals to be identical or different and,independently of one another, each to consist of 1 to 4 carbon atoms andto be straight-chain or branched, e.g.N,N′-dimethyl-para-phenylenediamine orN,N′-diethyl-para-phenylenediamine, hydroxylamines, e.g.N,N-diethylhydroxylamine, phosphorus-containing compounds, e.g.triphenylphosphine, triphenyl phosphite or triethyl phosphite, orsulfur-containing compounds, e.g. diphenyl sulfide or phenothiazine, ormixtures thereof.

Furthermore, they may also be degradation products or derivatives ofstabilizers, for example the Michael adduct of (meth)acrylic acid or(meth)acrylates and hydroquinone.

Stabilization can be effected in the presence or absence of molecularoxygen.

Preferably, the stabilization is effected using phenothiazine,hydroquinone, hydroquinone monomethyl ether,4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl,4-oxo-2,2,6,6-tetramethylpiperidin-N-oxyl, 2,6-di-tert-butyl-p-cresol ormixtures thereof in amounts of, in each case, from 10 to 5000 ppm.

The addition can be carried out in each case via the starting materialsor via the recycle or reflux streams.

In a particularly preferred manner, the dissolved stabilizer mixture issprayed onto condenser surfaces.

The catalysts used may be the known transesterification catalystsmentioned at the outset, preferably tetraalkyl titanates, alkali metalor alkaline earth metal alcoholates or metal chelates, particularlypreferably tetraalkyl titanates and very particularly preferably thetetraalkyl titanates of the alcohols participating in the reaction, e.g.Ti(OR¹)₄ or Ti(OR²)₄.

If a titanium alcoholate Ti(OR²)₄ is used, it can be prepared, forexample, as follows before use in the transesterification:

A titanium alcoholate Ti(OR⁴)₄ of a lower alcohol, preferably theisopropylate, isobutylate or n-butylate, is reacted with the higheralcohol R²OH (cf. equation 3) at elevated temperatures (50–130° C.). Thehigher alcohol is used in a molar excess (from 1:5 to 1:20). The loweralcohol R⁴OH is removed by distillation during the reaction.Ti(OR⁴)₄+R²OH

Ti(OR²)+R⁴OH  Equation 3For R², cf equation (1)

R⁴ is C₁–C₈-alkyl, preferably isopropyl, isobutyl or n-butyl R²OH andR⁴OH should preferably fulfill the following condition with regard totheir boiling points bp.:bp. (R²OH)≧bp. (R⁴OH)+20° C.

Under these conditions, it is technically simple to keep the losses ofR²OH low and to separate off R⁴OH as completely as possible.

The alkanol R⁴OH formed in the reaction is separated off by distillationor rectification, if required under reduced pressure. This can, ifrequired, be supported by stripping with a suitable unreactive gas. Theresulting residue is a catalyst solution for transesterification (Ticontent: 1–10% by weight) and as a rule contains less than 400 ppm ofR⁴OH. Thus, virtually no foreign alkanol (R⁴OH) is introduced into thetransesterification mixture and, as a rule, the content in the mixtureis less than 100 ppm.

However, it is of course also possible for mixed titanium alcoholates tobe present in the catalyst solution, depending on thetransesterification according to equation 3.

Transesterification

The lower (meth)acrylate I used has, as a rule, the followingcomposition:

-   -   99.0–99.95% by weight (meth)acrylate    -   0.001–0.1% by weight acetic ester    -   0.02–0.1% by weight propionic ester    -   0.001–0.05% by weight water

Furthermore, the lower alkanol R¹OH, its dialkyl ether, (meth)acrylicacid and other, for example isomeric (meth)acrylates may be present.

The higher alcohol R²OH usually has a purity of not less than 99.0% byweight and a water content of 0.01–0.2% by weight.

The content of ethylene glycol in the higher alcohol R²OH should not bemore than 100 ppm, preferably not more than 50 ppm, particularlypreferably not more than 20 ppm, in particular not more than 10 ppm. Thecontent of vinyloxyethanol in the higher alcohol R²OH should be not morethan 100 ppm, preferably not more than 50 ppm, particularly preferablynot more than 20 ppm, in particular not more than 10 ppm.

Mixtures of higher alkanols may also be used for thetransesterification.

The lower (meth)acrylate I is reacted with the higher alkanol R²OH in amolar ratio of from 1:1 to 4:1 in the presence of at least one of thecatalysts described above.

The transesterification is carried out in a reactor or a plurality ofreactors, preferably in one reactor or two reactors connected in series,having at least one attached rectification column and, if required,condensers of conventional design. In a cascade, each reactor may have adistillation or rectification column or the vapor phases of a pluralityof reactors are passed into at least one common distillation orrectification column.

Within a cascade, the temperature in the individual containers can beidentical or different; preferably, it increases in the course of thecascade. The pressure, too, may be identical or different in the courseof the cascade; it preferably decreases.

In general, the reaction temperature is 60–160° C., preferably 80–140°C., particularly preferably 100–130° C., and the total residence time is1–10, preferably 1–8, particularly preferably 2–6, in particular 2–4,hours.

The reaction zone can preferably be flushed continuously with a gas orgas mixture which is inert under the reaction conditions, e.g. nitrogen,air, nitrogen/oxygen mixtures, argon, helium, carbon dioxide or carbonmonoxide. Particularly preferably, the purge gas is passed along theheat exchanger surfaces present, in particular in a pumped or naturalcirculation present, as described in the German Patent Application withthe file reference DE 101 27 938.8.

A further preferred embodiment of the transesterification may comprisefeeding the reaction mixture continuously into a downstream reactorwhich is connected on the gas side to the transesterification reactor orthe attached column, as described in the same document.

The temperature in the downstream reactor is 1–10° C. higher than in thereactor.

The rectification columns are of a known design and have internalshaving separation activity (e.g. bubble trays, Thormann trays, valvetrays, sieve trays or dual-flow trays) or contain dumped or stackedpackings. The column(s) attached to the reactor(s) has/have as a rule10–30 theoretical plates. The reflux ratio is as a rule 5–15:1,preferably 7–12:1. The condensers are likewise of a known design, e.g.tubular or plate heat exchangers.

The uniform thorough mixing of the reaction solution is effected in aknown manner, for example by stirring, pumped circulation or naturalcirculation. The heat can be supplied, for example, via double-jacketheating and/or external or internal heat exchangers, e.g. tube-bundle orplate heat exchangers.

The lower alkanol liberated during the transesterification is removedfrom the reaction zone in vapor form via an attached distillation columnand, if required, is condensed with the aid of a conventional condenser,for example a tube-bundle or plate condenser. The cooling medium usedmay be, for example, water, air or brine.

The process is preferably carried out by separating off the loweralkanol R¹OH liberated in the transesterification, substantiallytogether with lower (meth)acrylate I and possibly higher alcohol R²OH.

The distillation conditions for this purpose are established so that thealkanol content in the condensate is 20–40% by weight in the case ofmethanol, 30–65, preferably 40–60, % by weight in the case of ethanoland 70–95, preferably 80–90, % by weight in the case of n-butanol. As arule, not more than 1, preferably not more than 0.5, particularlypreferably not more than 0.3, % by weight of the higher alcohol R²OH arecontained.

In a particularly preferred embodiment, the distillation conditions, forexample theoretical plates and reflux ratio, are chosen so that anonazeotropic mixture is taken off at the top of the column, in whichmixture the content of lower (meth)acrylate is higher compared with theazeotropic composition comprising lower alkanol and lower(meth)acrylate.

The condensate can be stabilized using a solution of at least onestabilizer (see above) in the abovementioned amount, preferably byspraying onto the condenser surfaces.

A part of the distillate, for example 50–95% by weight, can be recycledas reflux to the distillation column, and the remaining part, forexample from 5 to 50% by weight, is fed, according to the invention, tothe preparation of the lower alkyl (meth)acrylate. Preferably, from 60to 95, particularly preferably from 80 to 95, % by weight are added asreflux to the distillation column and from 5 to 40, particularlypreferably from 5 to 20, % by weight are fed to the preparation of thelower alkyl (meth)acrylate.

Of course, the distillate in vapor form can be passed, even withoutcondensation or after only partial condensation, into the preparation ofthe lower alkyl (meth)acrylate.

The distillate separated off during the transesterification can be fedcontinuously to a process for the preparation of the lower alkyl(meth)acrylate; particularly preferably, it is fed to the working-upprocess, but it can also be collected separately and fed in batchwise orsemicontinuously.

For example, the distillate separated off during the transesterificationcan be fed directly into the esterification reaction for the preparationof the lower alkyl (meth)acrylate. Since this is generallyacid-catalyzed, the small amount of the higher, basic alcohol isprotonated and then no longer participates in the esterificationreaction. This feed is preferred if only a very small amount of higheralkanol is present in the distillate, for example less than 1,preferably less than 0.5, particularly preferably less than 0.3, % byweight.

A working-up process is understood as meaning the measures which arerequired for purifying the lower (meth)acrylate after leaving thereaction zone together with any attached distillation column. Thiscomprises, for example, distillation, rectification and extractionsteps.

The distillate originating from the transesterification can, forexample,

-   -   be fed to a rectification process in which the lower alkanol        R¹OH is separated from the lower (meth)acrylate, for example the        separation of low boilers from the esterification mixture freed        from the acids in the preparation of the lower (meth)acrylate,        or    -   a wash process, for example for removing lower alkanol,        (meth)acrylic acid and/or acidic esterification catalyst from an        organic phase by means of an aqueous phase, e.g. water or alkali        solution, or    -   be used for the back-extraction of (meth)acrylic acid from        aqueous phases obtained in the working-up process, for example        from an aqueous phase, e.g. an esterification waste liquor which        is obtained on separating (meth)acrylic acid from the        esterification mixture or an aqueous phase which is obtained        after phase separation of the distillate in a distillation        within the working-up process.

The working-up process of the esterification can of course also becarried out in the presence of an assistant for the azeotropicdistillation, for example benzene, toluene or cyclohexane.

The aqueous phases used for the back-extraction can advantageously beacidified, for example with sulfuric acid, before the back-extraction.

The aqueous phases remaining after the washing or the back-extractioncan be further worked up, for example repeatedly subjected to the novelprocess or distilled or stripped for separating off lower alcohol orother useful products contained therein, or disposed of in aconventional manner.

The distillate originating from the transesterification is preferablyused for the back-extraction of (meth)acrylic acid from aqueous phaseswhich are obtained in the working-up process of the esterification andmay have been combined, particularly preferably in the back-extractionof (meth)acrylic acid from wash water resulting during the treatment ofthe esterification reactor discharge with an aqueous phase.

For this purpose, one part by weight of the aqueous phase to beextracted is treated with, for example, from 0.1 to 5, preferably from0.1 to 3, particularly preferably from 0.15 to 2, in particular from 0.2to 1.5, parts by weight of the distillate originating from thetransesterification.

The distillate can be used as such for the back-extraction, but loweralkanol R¹OH and/or lower alkyl (meth)acrylate I can also be added, sothat the mixing ratio in the extracting agent is substantially 1 to 20parts by weight of lower alkanol to 1 to 5 parts by weight of loweralkyl (meth)acrylate. Such an addition to the distillate originatingfrom the transesterification is generally advantageous when the contentof lower alkyl (meth)acrylate is less than 30% by weight.

The extraction is carried out in general at temperatures between thehighest melting point and the lowest boiling point of the componentspresent in the system, for example at from 0 to 80° C., preferably from0 to 60° C., particularly preferably from 10 to 50° C.

In terms of process engineering, all known extraction processes andextraction apparatuses may be used for an extraction according to thenovel process, for example those which are described in Ullmann'sEncyclopedia of Industrial Chemistry, 6th ed, 1999 Electronic Release,Chapter: Liquid—Liquid Extraction—Apparatus. For example, the extractionmay be a one-stage or multistage, preferably multistage, extraction orone effected by the cocurrent or countercurrent procedure, preferablythe countercurrent procedure.

Sieve tray columns, columns containing stacked or dumped packings,mixer-settler apparatuses and columns having rotating internals arepreferably used.

In a further preferred embodiment, any desired (meth)acrylicacid-containing organic esterification mixture obtained in thepreparation of the lower alkyl (meth)acrylate is mixed with at least apart of the organic phase of the distillate obtained during thetransesterification which was not used as reflux for the distillationcolumn. This can be effected, for example, before or during washing ofthe (meth)acrylic acid-containing stream with water and/or aqueousalkali solution.

In a further preferred embodiment, the wastewaters of this acidseparation which, if required, may also be carried out several times,can be combined and acidified (pH<3), for example with 20–60% strengthsulfuric acid. According to the invention, the (meth)acrylic acid canthen be extracted in accordance with DE 23 23 328 with a mixture oflower alcohol and lower alkyl (meth)acrylate, e.g. butanol and butylacrylate, this back-extraction being effected according to the inventionat least partly with the distillate originating from thetransesterification.

The (meth)acrylic acid-containing organic phase (extract) is feddirectly to a reactor, preferably the first reactor, of the reactionzone of the esterification process, preferably via any attacheddistillation column, the feed preferably being effected in the lowerhalf of the column.

The resulting wastewaters of the preparation process of theesterification can be combined and the lower alcohol dissolved thereincan be recovered by stripping with steam or by distillation and/or canbe disposed of.

The further working-up of the reaction mixture of thetransesterification has substantially no effect on the novel process.

It can be carried out, for example, as described in the German PatentApplications with the file references DE 101 27 939.6 and DE 101 27938.8.

According to the invention, the preparation of the lower (meth)acrylateis not limited and can be carried out, for example, by one of theprocesses mentioned at the outset, for example according to DE-A1 198 51983. The process described there substantially comprises washing thereactor discharge of an esterification reaction, substantially composedof desired ester, (meth)acrylic acid, low boilers, catalyst andoxyesters, with water and/or aqueous alkali solution, the catalyst andthe unconverted (meth)acrylic acid being separated off substantiallycompletely.

Dialkylaminoalkyl (meth)acrylates prepared according to the invention,in particular dialkylaminoethyl (meth)acrylates and especiallydimethylaminoethyl (meth)acrylates, are useful monomers for thepreparation of copolymers. They are used as monomers in thepolymerization in the present form or after quaternization.

Conventional quaternizing agents are, for example, benzyl halides, e.g.benzyl chloride, alkyl halides, e.g. methyl chloride, ethyl chloride,methyl bromide, ethylene dichloride or allyl chloride, alkylene oxides,e.g. ethylene oxide, propylene oxide, styrene oxide, isobutylene oxideor vinyloxirane, preferably ethylene oxide or propylene oxide,particularly preferably ethylene oxide, alkyl phosphites orphosphonates, e.g. trimethyl phosphite or triethyl phosphite, dialkylsulfates, e.g. dimethyl sulfate or diethyl sulfate, dialkyl carbonates,e.g. dimethyl carbonate, diethyl carbonate or di-n-butyl carbonate,chlorohydrin or epichlorohydrin.

In particular, those copolymers which contain the quaternized monomersincorporated as polymerized units are used in water treatment, forexample as ion exchange resins or as a component of membranes.

The examples which follow illustrate the novel process withoutrestricting it.

ppm and percentages used in this document are percentages by weight andppm by weight, unless stated otherwise.

EXAMPLE 1 Transesterification Example

A mixture of 3600 g of n-butyl acrylate, 1500 g of dimethylaminoethanol,100 g of titanium tetra-n-butylate, 3 g of hydroquinone monomethyl etherand 1 g of phenothiazine was heated to the boil in a 10 l stirredreactor having an attached packed column (height 150 cm, diameter 2.8cm, 0.5 cm Raschig rings as packing) and condenser. The n-butanolliberated was separated off in gaseous form via the column and wascondensed (top temperature 92° C., 370 mbar). 1535 g of distillate weredischarged in the course of 4 hours. 100 g per hour of a solution of1000 ppm of phenothiazine in n-butyl acrylate were added at the top ofthe column. The distillate had substantially the following composition:

-   -   79.5% by weight n-butanol    -   20.1% by weight n-butylacrylate    -   0.2% by weight dimethylaminoethanol

According to gas chromatographic analysis, the reaction mixture (4060 g)substantially contained

-   -   56.3% by weight dimethylaminoethyl acrylate    -   40.1% by weight n-butyl acrylate    -   0.6% by weight dimethylaminoethanol    -   1.8% by weight n-butanol    -   0.8% by weight oxyester

The conversion with respect to dimethylaminoethanol was 98% and theyield 95%.

EXAMPLE 2 Extraction Example

The waste liquors obtained in the course of the separation of acid fromthe reaction discharge in the preparation of n-butyl acrylate accordingto DE-A 198 51 983, example 2, were combined and were acidified withsulfuric acid (pH 1), and the acrylic acid (content 3.24% by weight) wasextracted in one stage in a separating funnel at 25° C. with thedistillate from example 1, separated off from the esterificationreactor, or with a distillate enriched with n-butyl acrylate or withmixtures of n-butanol and n-butyl acrylate (in each case 25 g/100 g ofwaste liquor).

The amount of extracted acrylic acid in the waste liquor was determined.Degree of extraction=(content of acrylic acid before extraction−contentof acrylic acid after extraction)/content of acrylic acid beforeextraction

The distillate from example 1, obtained during the transesterificationof butyl acrylate with dimethylaminoethanol, had substantially thefollowing composition:

-   -   79.5% by weight n-butanol    -   20.1% by weight n-butyl acrylate    -   0.2% by weight dimethylaminoethanol

For comparison, synthetic mixtures of n-butanol and n-butyl acrylatehaving the following weight ratios were used:

Extracting agent: Degree of extraction: distillate (see above) 35%distillate + n-butyl acrylate 1:0.6 45% n-butanol:n-butyl acrylate 1:344% n-butanol:n-butyl acrylate 1:1 45% n-butanol:n-butyl acrylate 3:138% n-butanol:n-butyl acrylate 4:1 36%

After the extraction, no dimethylaminoethanol was detectable in theextracting agent.

EXAMPLE 3 Esterification Example

A stirred kettle cascade consisting of 3 stirred reactors which each hada reaction volume of 1 l and were equipped with column, condenser andphase separation vessel was fed in continuous operation with, per hour,533 g of crude acrylic acid, 15 g of sulfuric acid, 510 g of n-butanoland 200 g of the mixture obtained in the back-extraction of the acrylicacid (see below). In addition, 107 g of a low boiler fraction (seebelow) per hour were recycled via the column of the first reactor. Thecrude acrylic acid contained substantially 99.3% by weight of acrylicacid, 0.2% by-weight of acetic acid, 0.03% by weight of propionic acid,0.11% by weight of maleic anhydride, 0.2% by weight of diacrylic acidand 0.1% by weight of phenothiazine. The reaction temperatures in thereactors were 107° C., 118° C. and 125° C., and the pressure was 700mbar. A mixture of water, n-butanol, n-butyl acrylate, n-butyl acetateand dibutyl ether was obtained at the top of the column and separatedinto an aqueous phase and an organic phase. The aqueous phase wasdischarged and, apart from the discharged fraction (see below), theorganic phase was recycled as reflux to the column. 25 g per hour of theorganic phase, mainly comprising 24% by weight of n-butyl acrylate, 54%by weight of n-butanol, 8% by weight of n-butyl acetate and 2% by weightof dibutyl ether, were discharged. The column was stabilized by adding30 g of a 1% strength phenothiazine solution in n-butyl acrylate to theuppermost tray. The condensate was stabilized by applying 50 ml of a 1%strength aqueous solution of4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl to the condenser. Thereactor discharge (1280 g/h) still contained 0.6% by weight of acrylicacid, and the acrylic acid conversion was 98.0%.

The reactor discharge cooled to about 25° C. was washed in one-stagemixer-settler apparatuses with a 6% strength by weight sodium hydroxidesolution (200 ml/h) and with water (100 ml/h) to separate off acid.

The substantially acid-free organic phase was separated into a lowboiler fraction and a bottom product, which mainly contained n-butylacrylate and high boilers, in a distillation unit consisting of acirculation evaporator, a column having 40 bubble trays, a condenser anda phase separation vessel. 107 g of the low boiler fraction, mainlycomprising butyl acrylate (63.6% by weight), butyl acetate (1.3% byweight), butanol (26% by weight) and dibutyl ether (0.5% by weight),were recycled into the esterification cascade. The reflux ratio was 10.The bottom temperature was 107° C. and the top temperature was 80° C. at175 mbar. The distillation unit was stabilized by applying 50 g of 1%strength by weight solution of phenothiazine in n-butyl acrylate to thecondenser.

In a further distillation unit consisting of a circulation evaporator, adual-flow column (30 trays) and a condenser, n-butyl acrylate in apurity of 99.9% was separated from the bottom product (yield 95%). Thefeed was effected on the 10th tray, the bottom temperature was 110° C.and the top temperature was 80° C. at 105 mbar. The column wasstabilized with 0.1% strength by weight hydroquinone monomethyl ether inn-butyl acrylate via the reflux (15 ppm of hydroquinone monomethylether, reflux ratio 0.4) and by adding a 1% strength by weight solutionof hydroquinone monomethyl ether in n-butyl acrylate to the 15th tray.

The aqueous phases obtained on separating off acid were combined,acidified (pH 1) with concentrated sulfuric acid and extracted with amixture of 120 g/h of distillate from the transesterification analogousto example 1 and 80 g of n-butyl acrylate in a three-stage mixer-settlerapparatus. The acrylic acid-containing organic phase in which nodimethylaminoethanol was detectable (3% by weight of acrylic acid) wasrecycled to the first esterification reactor.

The esterification could be operated without problems for at least 500hours without dimethylaminoethyl acrylate being detectable in theproduct.

1. A process for the preparation of higher (meth)acrylates bytransesterification of a lower (meth)acrylate with a higher alcohol R²OHwhich either carries, as R², C₂–C₁₂-alkyl radical having at least oneNR³ ₂ group in which R³ is C₁–C₆-alkyl and N may also be a member of afive- to seven-membered ring or is R³ ₂N(—CH₂CH₂—O)y-H, R³₂N(—CH(CH₃)—CH—O)_(y)—H or R³ ₂N(—CH₂CH(CH₃)—O)_(y)—H, where y is aninteger from 1 to 4, in the presence of a stabilizer or stabilizermixture and of a catalyst or catalyst mixture, wherein the liberatedlower alkanol R¹OH, where R¹ contains at least 1 carbon atom less thanR², is separated off and is fed at least partly to the preparation of alower (meth)acrylate without further purification.
 2. A process for thepreparation of higher (meth)acrylates by transesterification of a lower(meth)acrylate with a higher alcohol R²OH which is selected from2-(dimethylamino)ethanol, 3-(dimethylamino)propanol,4-(dimethylamino)butanol, 5-(dimethylamino)pentanol,6-(dimethylamino)hexanol, 8-(dimethylamino)octanol,10-(dimethylamino)decanol, 12-(dimethylamino)dodecanol,2-(diethylamino)ethanol, 3-(diethylamino)propanol,4-(diethylamino)butanol, 5-(diethylamino)pentanol,6-(diethylamino)hexanol, 8-(diethylamino)octanol,10-(diethylamino)decanol, 12-(diethylamino)dodecanol,2-(di(isopropyl)amino)ethanol, 3-(di(isopropyl)amino)propanol,4-(di(isopropyl)amino)-butanol, 5-(di(isopropyl)amino)pentanol,6-(di(isopropyl)amino)hexanol, 8-(di(isopropyl)amino)octanol,10-(di(isopropyl)amino)decanol, 12-(di(isopropyl)amino)-dodecanol,2-(dibutylamino)ethanol, 3-(dibutylamino)propanol,4-(dibutylamino)butanol, 5-(dibutylamino)pentanol,6-(dibutylamino)hexanol, 8-(dibutylamino)octanol,10-(dibutylamino)decanol, 12-(dibutylamino)dodecanol,2-(dihexylamino)ethanol, 3-(dihexylamino)propanol,4-(dihexylamino)butanol, 5-(dihexylamino)pentanol,6-(dihexylamino)hexanol, 8-(dihexylamino)octanol,10-(dihexylamino)decanol, 12-(dihexylamino)dodecanol,2-(methylethylamino)ethanol, 2-(methylpropylamino)ethanol,2-(methylisopropylamino)-ethanol, 2-(methylbutylamino)ethanol,2-(methylhexylamino)ethanol, 2-(methyloctylamino)ethanol,2-(ethylpropylamino)ethanol, 2-(ethylisopropylamino)ethanol,2-(ethylbutylamino)ethanol, 2-(ethylhexylamino)ethanol,2-(ethyloctylamino)ethanol, 3-(methylethylamino)propanol,3-(methylpropylamino)propanol, 3-(methylisopropylamino)-propanol,3-(methylbutylamino)propanol, 3-(methylhexylamino)-propanol,3-(methyloctylamino)propanol, 3-(ethylpropylamino)-propanol,3-ethylisopropylamino)propanol, 3-(ethylbutylamino)propanol,3-(ethylhexylamino)propanol, 3-(ethyloctylamino)propanol,4-(methylethylamino)butanol, 4-(methylpropylamino)butanol,4-(methylisopropylamino)-butanol, 4-(methylbutylamino)butanol,4-(methylhexylamino)butanol, 4-(methyloctylamino)butanol,4-(ethylpropylamino)butanol, 4-(ethylisopropylamino)butanol,4-(ethylbutylamino)butanol, 4-(ethylhexylamino)butanol,4-(ethyloctylamino)butanol, 2-(N-piperidinyl)ethanol,3-(N-piperidinyl)propanol, 4-(N-piperidinyl)butanol,5-(N-piperidinyl)pentanol, 6-(N-piperidinyl)hexanol,8-(N-piperidinyl)octanol, 10-(N-piperidinyl)decanol,12-(N-piperidinyl)dodecanol, 2-(N-pyrrolidinyl)ethanol,3-(N-pyrrolidinyl)propanol, 4-(N-pyrrolidinyl)butanol,5-(N-pyrrolidinyl)pentanol, 6-(N-pyrrolidinyl)hexanol,8-(N-pyrrolidinyl)octanol, 10-(N-pyrrolidinyl)decanol,12-(N-pyrrolidinyl)dodecanol, 2-(N-morpholino)ethanol,3-(N-morpholino)propanol, 4-(N-morpholino)butanol,5-(N-morpholino)pentanol, 6-(N-morpholino)hexanol,8-(N-morpholino)octanol, 10-(N-morpholino)decanol,12-(N-morpholino)dodecanol, 2-(N′-methyl-N-piperazinyl)-ethanol,3-(N′-methyl-N-piperazinyl)propanol, 4-(N′-methyl-N-piperazinyl)butanol,5-(N′-methyl-N-piperazinyl)pentanol, 6-(N′-methyl-N-piperazinyl)hexanol,8-(N′-methyl-N-piperazinyl)octanol, 10-(N′-methyl-N-piperazinyl)decanol12-(N′-methyl-N-piperazinyl)-dodecanol,2-(N′-ethyl-N-piperazinyl)ethanol, 3(N′-ethyl-N-piperazinyl)propanol,4-(N′-ethyl-N-piperazinyl)butanol, 5-(N′-ethyl-N-piperazinyl)pentanol,6-(N′-ethyl-N-piperazinyl)hexanol, 8-(N′-ethyl-N-piperazinyl)octanol,10-(N′-ethyl-N-piperazinyl)decanol,12-(N′-ethyl-N-piperazinyl)dodecanol,2-(N′-isopropyl-N-piperazinyl)ethanol,3-(N′-isopropyl-N-piperazinyl)-propanol,4-(N′-isopropyl-N-piperazinyl)butanol,5-(N′-isopropyl-N-piperazinyl)pentanol,6-(N′-isopropyl-N-piperazinyl)hexanol,8-(N′-isopropyl-N-piperazinyl)octanol,10-(N′-isopropyl-N-piperazinyl)decanol,12-(N′-isopropyl-N-piperazinyl)dodecanol or ethoxylated and/orpropoxylated alcohols and mixed ethoxylated/propoxylated amino alcohols,R³ ₂N(—CH₂CH₂—O)_(y)—H or R³ ₂N(—CH(CH₃)—CH₂—O)_(y)—H or R³₂N(—CH₂—CH(CH₃)—O—)_(y)—H, where y is an integer from 1 to 4, in thepresence of a stabilizer or stabilizer mixture and of a catalyst orcatalyst mixture, wherein the liberated lower alkanol R¹OH, which isselected from methanol, ethanol, n-propanol, isopropanol, n-butanol,isobutanol, 2-butanol and tert-butanol, is separated off and is fed atleast partly to the preparation of a lower (meth)acrylate withoutfurther purification.
 3. A process as claimed in claim 2, wherein thehigher alcohol is selected from dimethylaminoethanol,diethylaminoethanol, di-n-butylaminoethanol, 3-dimethylaminopropanol,3-diethylaminopropanol and 3-di-n-butylaminopropanol.
 4. A process asclaimed in claim 1, wherein the liberated lower alkanol R¹OH isseparated off and is fed at least partly to a working-up process in thepreparation of the lower (meth)acrylate.
 5. A process as claimed inclaim 1, wherein the lower alkanol R¹OH is separated off bydistillation.
 6. A process as claimed in claim 1, wherein the loweralkanol R¹OH liberated during the transesterification is separated offsubstantially together with lower (meth)acrylate and any higher alcoholR²OH.
 7. A process as claimed in claim 1, wherein the mixture liberatedduring the transesterification, separated off and containing loweralkanol R¹OH is brought into contact with a (meth)acrylicacid-containing stream in a working-up process in the preparation of thelower (meth)acrylate.
 8. A process as claimed in claim 7, wherein themixture liberated during the transesterification, separated off andcontaining lower alkanol R¹OH is used in a working-up process in thepreparation of the lower (meth)acrylate for an extraction of(meth)acrylic acid.
 9. A process as claimed in claim 7, wherein themixture liberated during the transesterification, separated off andcontaining lower alkanol R¹OH is used in the working-up process in thepreparation of a lower (meth)acrylate for an extraction of (meth)acrylicacid from acidified washwater.
 10. A process as claimed in claim 8,wherein the extraction is carried out countercurrently.
 11. A process asclaimed in claim 1, which is carried out continuously.
 12. A process asclaimed in claim 1, wherein the lower alkanol R¹OH is n-butanol and thelower (meth)acrylate is n-butyl (meth)acrylate.
 13. A process as claimedin claim 1, wherein the higher alkanol R²OH is2-(N,N-dimethylamino)ethanol and the higher (meth)acrylate is2-(N,N-dimethylamino)ethyl (meth)acrylate.
 14. A process as claimed inclaim 2, wherein the liberated lower alkanol R¹OH is separated off andis fed at least partly to a working-up process in the preparation of thelower (meth)acrylate.
 15. A process as claimed in claim 2, wherein thelower alkanol R¹OH is separated off by distillation.
 16. A process asclaimed in claim 2, wherein the lower alkanol R¹OH liberated during thetransesterification is separated off substantially together with lower(meth)acrylate and any higher alcohol R²OH.
 17. A process as claimed inclaim 2, wherein the mixture liberated during the transesterification,separated off and containing lower alkanol R¹OH is brought into contactwith a (meth)acrylic acid-containing stream in a working-up process inthe preparation of the lower (meth)acrylate.
 18. A process as claimed inclaim 17, wherein the mixture liberated during the transesterification,separated off and containing lower alkanol R¹OH is used in a working-upprocess in the preparation of the lower (meth)acrylate for an extractionof (meth)acrylic acid.
 19. A process as claimed in claim 17, wherein themixture liberated during the transesterification, separated off andcontaining lower alkanol R¹OH is used in a working-up process in thepreparation of the lower (meth)acrylate for an extraction of(meth)acrylic acid from acidified washwater.
 20. A process as claimed inclaim 18, wherein the extraction is carried out countercurrently.
 21. Aprocess as claimed in claim 2, which is carried out continuously.
 22. Aprocess as claimed in claim 2, wherein the lower alkanol R¹OH isn-butanol and the lower (meth)acrylate is n-butyl (meth)acrylate.
 23. Aprocess as claimed in claim 2, wherein the higher alkanol R²OH is2-(N,N-dimethylamino)ethanol and the higher (meth)acrylate is2-(N,N-dimethylamino)ethyl (meth)acrylate.
 24. A process for thepreparation of higher (meth)acrylates by transesterification of a lower(meth)acrylate with a higher alcohol R²OH which either carries, as R², aC₂–C₁₂-alkyl radical having at least one NR³ ₂ group in which R³ isC₁–C₆-alkyl and N may also be a member of a five- to seven-membered ringor is R³ ₂N(—CH₂CH₂—O)y-H, R³ ₂N(-CH(CH₃)—CH—O)_(y)—H or R³₂N(—CH₁₂CH(CH₃)—O)_(y) 13 H, where y is an integer from 1 to 4, in thepresence of a stabilizer or stabilizer mixture and of a catalyst orcatalyst mixture, wherein the liberated lower alkanol R¹OH, where R¹contains at least 1 carbon atom less than R², is separated off and isfed at least partly to the preparation of a lower (meth)acrylate withoutfurther purification over ion-exchange resin.